IP Multimedia Subsystem (IMS) is a technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over mobile communication networks. IMS provides features to enrich the end-user communication experience by integration and interaction of services. The IMS uses the Session Initiation Protocol (SIP) to set up and control a call.
Network components involved in the call are collectively referred to as the connection between terminating sides of the call. The Session Description Protocol (SDP), which is carried by a SIP message, is used to describe and negotiate media components for the connection. For media transmission, a data protocol different from a control protocol is used along a media path that at least partially deviates from a signal path carrying the SIP messages.
The connection extends along a plurality of nodes, a multiplicity of which is adapted to insert audio announcements, e.g., speech or tones. For example, the multiplicity of nodes or all nodes have to consistently update their internal status. In some cases, each of the multiplicity of nodes may insert an announcement in the media path, e.g., responsive to the status update.
A connection including multiple nodes capable of playing announcements can be due to service centralization requiring that the connection remains anchored at a certain node of the network, so that a mobile connection extends along an increased number of nodes of the network after a User Equipment (UE) is handed-over to another type of access network.
As an example for a hand-over between different types of access networks, before 4G network coverage according to 3GPP Long Term Evolution (LTE) is sufficient to allow 2G and 3G networks to be completely replaced, a call initiated using a Packet-Switched (PS) network access must be handed-over to a Circuit-Switched (CS) network access provided by a 2G network according to the Global System for Mobile communications (GSM) or a 3G network according to the Universal Mobile Telecommunications System (UMTS), which use GERAN and UTRAN, respectively, as radio access network technology. WO 2012/062379 describes a connection including a signal path and a media path that change as the UE is moving out of LTE coverage.
IMS Centralized Services (ICS), as specified in standard document 3GPP TS 23.292, version 12.1.0, allows IMS sessions using CS bearers to be treated as standard IMS sessions. ICS defines signaling mechanism between the UE and the IMS for transport of information to centralize the services in the IMS. Standard document 3GPP TS 23.237, version 12.4.0, defines additional procedures needed for service continuity of media transport when handing over to the CS network access. ICS thus allows for delivery of consistent IMS services to the user regardless of the attached access type. Services and service control are based on IP Multimedia (IM) Core Network (CN) subsystem mechanisms, while using CS access for the media bearer.
In previous 2G and 3G networks, basic call and supplementary services were always executed in the mobile access node. According to the standard document 3GPP TS 24.292, version 12.1.0, connections originating from an ICS subscriber using both the IM CN subsystem and the CS domain are anchored in the IM CN subsystem. This results in a spreading of the basic and supplementary call-related services among the access network node, to which the mobile ICS subscriber is wirelessly coupled, and the IMS service network, e.g., by spreading the connection over access network and IMS service network. Consequently, both the access network and the IMS service network provide nodes capable of playing announcements. The service centralization in the IMS can be implemented by a Service Centralization and Continuity Application Server (SCC AS) and a Multimedia Telephony Application Server (MMTel AS).
Since it is difficult to replace an existent mobile access network by an ICS network providing the same level of service, ICS is introduced stepwise in networks providing in parallel 2G and 3G access. At the beginning of the ICS introduction, the basic call and supplementary services are executed either in the access network node, to which the ICS subscriber is coupled, or in the IMS service network. As a final target of the ICS introduction, the execution of all services is concentrated in the IMS network.
As long as the service concentration does not reach this final target and services are executed in more than one node in the network, events that trigger an announcement can be detected at more than one node of the network. This may cause uncoordinated, especially status-related events, (e.g., parallel, overlapping or mutually interrupted) speech announcements and tone playing by multiple nodes.